1. Field of the Invention
The present invention relates to a liquid crystal display device using the guest-host mode.
2. Description of the Related Art
Recently, a rapid progress has been made in the application of liquid crystal display devices to word processors, laptop personal computers, and pocket TV sets. In particular, liquid crystal display devices of reflection type, which work with reflection of incident light, do not require a back light, thereby consuming a smaller amount of power and requiring less thickness and weight. Thus they attract attention from many people.
Conventionally, liquid crystal display devices of reflection type use the TN (twisted nematic) mode or the STN (super twisted nematic) mode. These modes involves mounting a linear polarizer therein to shield an incident light. As a consequence, it naturally follows that a half of the intensity of incident light is not used in the display, which darkens the displayed image. To overcome such a drawback, a display mode for effectively using all the incident light has been proposed. Examples of such mode include the phase transition type guest-host mode (D. L. White and G. N. Taylor: J. Appl. Phys. 45, 4718, 1974). This mode uses the cholesteric-nematic phase transition phenomenon caused by an electric field. Into a liquid crystal cell subjected to a vertical aligning treatment is disposed a liquid crystal composition; comprising a cholesteric liquid crystal and a two-tone pigment, a cholesteric liquid crystal being obtained by adding a chiral material into a nematic liquid crystal having a positive dielectric anisotropy.
Even when a voltage is not applied, the liquid crystal composition forms a helix in accordance with the amount of added chiral materials. Consequently, the two-tone pigment absorbs light passing through the liquid crystal device, which develops a color. In the next step, application of a voltage more than the threshold value provides a homeotropic orientation, which allows light passing through the liquid crystal display device to develop no color without being absorbed by the two-tone pigment. In addition to this mode, a multi-tone display of reflection type is also proposed by combining a micro color filter(Proceedings of SID Vol. 29 157 1988).
By the way, liquid crystal display devices of reflection type, particularly color liquid crystal display devices of the same type having a reflection plate carried on the outside of the cell produce a parallax because of the glass thickness of the reflection plate, as while producing a shadow and an insufficient mixture of colors. This problem is settled by forming within the cell a reflection plate which also serves as a pixel electrode. Such pixel electrode controls a reflected light with the irregular patterns formed on the surface.
However, rendering uniform the orientation in the liquid crystal cell is extremely difficult. The display mode described in the above documents produces an irregular helix when the step of the recess formed in the reflection plate is larger than the thickness of the surface layer; hence such display mode produces an irregular display. Furthermore, with respect to liquid crystal display devices using a liquid crystal composition having a small ratio of d/P.sub.0 where P.sub.0 represents a helical pitch of the liquid crystal composition, and d a cell thickness of the devices, the reduced light absorption by the pigment at the application of no voltage results in a low contrast ratio although applying the low threshold voltage. Besides, when d/P.sub.0 is large, light absorption by the pigment at the application of no voltage increases to offer a high contrast. On the other hand, the threshold value increases, which is not suitable for a low-voltage drive.